This invention relates to the field of coating by which a plurality of viscous coating compositions may be curtain coated as a composite layer at high speed onto a continuously moving receiving surface, such as in the manufacture of photographic films and papers, magnetic recording tapes and such like.
Curtain coating methods for the simultaneous coating of multiple layers are well known. Such methods are described in U.S. Pat. No. 3,508,947 and U.S. Pat. No. 3,632,374. These documents emphasize the advantages of such methods for applying photographic compositions onto paper and polymeric substrates. The coating comprises multiple layers which are formed into a free falling curtain and allowed to impinge upon a continuously moving substrate. Important parameters to consider include the height h of the curtain, the application angle xcex8 between the horizontal and the tangent to the substrate at the point of impingement, measured on the upstream side of the curtain. WO 89/05477 describes how an electrostatic voltage may be applied at the coating point to avoid the problems of air entrainment.
The occurrence of a recirculating flow in curtain coating, sometimes called puddling, is well documented in the prior art. The phenomena of puddling occurs when the volumetric flow rate of the coating liquid is sufficiently high, and/or the substrate speed is sufficiently low, such that a bank of liquid forms at the upstream side of the falling curtain. The excess liquid is sometimes termed a heel. At extremes of high flow rate, and/or low substrate speed, recirculating eddies may be formed within the heel. These persistent recirculating eddies can trap air bubbles or particles and may disrupt the coating flow by prolonging the residence time of a particular coating layer within the heel. They may also lead directly to non-uniform laydown. When an eddy is present, the flow of liquid through the heel may develop a transverse velocity component along the length of the heel which may result in thickness variations of the final coating and/or interlayer mixing. Simultaneous multilayer coatings are a particular problem since the total flow rate is the sum of the individual flow rates for each layer and can rapidly become large as the number of layers increases. The ability to determine the limits within which coating parameters must be kept to avoid recirculation is therefore a considerable aid in reducing coating defects.
Heel formation without the presence of recirculating eddies is possible. It is therefore possible to obtain uniform coatings when a heel is present. Under these conditions flow lines within the heel remain smooth and continuous causing no disruption of the layers. Generally however this is also undesirable since the loss of momentum from the curtain may produce air entrainment at relatively low speeds. This is described in xe2x80x9cHydrodynamics of dynamic wettingxe2x80x9d Blake et al. A.I.Ch.E. Journal, 40, (1994), pp229.
A theoretical model describing when eddies occur in bead coating as a function of coating parameters such as Newtonian viscosity and surface tension can be found in Hens J., and Van Abbenyen W., xe2x80x9cSlide Coatingxe2x80x9d, in xe2x80x98Liquid film coatingxe2x80x99 1997 ISBN 0412064812. However there is no disclosure of any relation in curtain coating that accounts for the interactive effects of the relevant control parameters such as curtain height, application angle, electrostatic voltage and solution rheology (viscosity). It is known that for Newtonian solutions, increasing curtain height, increasing flow rate and reducing viscosity separately or in combination, promotes puddling. However the effects of shear thinning solution, application angle and electrostatic voltage are not known.
The widespread use of highly shear thinning coating solutions based on gelatin plus a polymeric thickener significantly complicates the issue since the viscosity of such a solution is a strong function of the applied shear rate. Predictions based on Newtonian solutions with constant viscosity give unreliable results when applied to highly shear-thinning (non-Newtonian) solutions of gelatin plus thickener. In order to determine the recirculation behavior of such solutions the effective shear rate within the heel region must be obtained, a value that cannot be measured directly. To determine whether or not a solution is xe2x80x98highly shear-thinningxe2x80x99 the following formula is used:                     η        =                                                            η                0                            -                              η                ∞                                                                    (                                  1                  +                                                            (                                              γ                                                  γ                          c                                                                    )                                        2                                                  )                                                              1                  -                  n                                2                                              +                      η            ∞                                              (        1        )            
xcex7 (cP) is the apparent viscosity of the solution when the applied shear rate is xcex3 (sxe2x88x921). The critical shear rate is xcex3c (sxe2x88x921), above which the solution viscosity begins to decrease from its low shear value xcex70 (cP) (xcex3 less than xcex3c), down to its limiting high shear value xcex7∞ (cP) (xcex3 greater than  greater than xcex3c). The rate that the viscosity decreases once the shear rate is greater than the critical shear rate, is determined by the power law index n. By fitting equation (1) to viscosity measurements taken over a range of shear rates values for xcex3c and n can be obtained.. For a Newtonian liquid, n equals 1, and for a shear thinning liquid n is less than 1; the smaller n, the more rapidly viscosity falls with increasing shear rate. In the following description a solution will be termed highly shear thinning if it has a power law index n less than 0.8 and a critical shear rate xcex3c less than 400sxe2x88x921.
The phenomena of air entrainment with recirculation, often referred to as xe2x80x98saggingxe2x80x99, is a restriction on the maximum attainable coating speed in any curtain coating operation. Various practical methods for avoiding sagging are known. EP 426,122(B 1) describes a range of preferred values for the angle of inclination of a hopper slide to the horizontal, and the angle between the falling curtain and a tangent to the substrate at the point of impingement, measured at the downstream side of the curtain. The solution viscosity is then selected to ensure a concave wetting line. The method specifically described in EP 426,122(B1) results in the formation of a heel with a degree of concavity of at least 3 mm, this being the distance measured from a straight line drawn between the edges of the curtain to the centre of the wetting line. No mention is made of the presence or absence of recirculation within the heel. For selection of an appropriate solution viscosity at flow rates  less than 4.0 cm2/s, reference is made to JP 1131549, which also describes a preferred range for the hopper slide angle and a minimum viscosity of 40 cP for the bottom layer of a two layer coating to avoid xe2x80x98turbulencexe2x80x99 in the coating. The term turbulence in the context of JP 1131549 is taken to be the phenomena termed recirculation in this specification. Neither EP 426,122(B 1) nor JP 1131549 disclose the interaction of viscosity, flow rate, substrate speed, application angle or curtain height and the propensity for heel formation. The effects of using shear-thinning coating solutions are similarly unspecified, with the relevant solution viscosity assumed to be that measured at low shear rates of 10-30sxe2x88x921. EP 836,117(A2) mentions specifically the lack of unified understanding of heel formation in curtain coating and the interaction of key coating parameters. U.S. Pat. No. 5,393,571 specifies a preferred range of hopper slide angle and a minimum value of 90 cP for the viscosity at 10sxe2x88x921 of the coating liquid, in conjunction with a minimum substrate roughness of 0.3 xcexcm. Exactly which roughness parameter is to exceed 0.3 xcexcm is not specified making the definition of little practical benefit.
The aim of the present invention is to provide a method which avoids the presence of a recirculating heel that may cause coating non-uniformities or reduce the maximum attainable coating speed.
According to the present invention there is provided a method of curtain coating which avoids coating defects due to recirculation, the curtain being formed from at least one layer of coating solution having a composite density xcfx81 (kgmxe2x88x923) and a total volumetric flow rate per unit curtain width Q (m2sxe2x88x921), the curtain being allowed to free fall a distance h (m), at a velocity U (msxe2x88x921), onto a continuously moving substrate having a velocity S (msxe2x88x921) with an application angle of xcex8 between the horizontal and tangent to the substrate at the point of impingement, the dynamic surface tension at the rear of the falling curtain being "sgr" (mNmxe2x88x921), the aforementioned variable parameters being controlled so as to satisfy the following inequality;
xe2x80x83We less than 7.82.(Ca)0.39
where       We    =                            ρ          ⁢                      xe2x80x83                    ⁢          QU          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢          θ                          σ          -                      α            ⁢                          xe2x80x83                        ⁢                          F              x                                          ⁢              xe2x80x83            ⁢      and                  Ca      =                        η          ⁡                      (                          S              +                              U                ⁢                                  xe2x80x83                                ⁢                sin                ⁢                                  xe2x80x83                                ⁢                θ                                      )                                    σ          -                      α            ⁢                          xe2x80x83                        ⁢                          F              x                                            ,  
recirculation being avoided if the above inequality is satisfied.
Preferably We less than 4.82.Ca0.39 
All of the methods suggested in the prior art to avoid the problems associated with heel formation are based on adjustment of a small number of parameters to improve the coating speed/uniformity in the specified situation. Thus a range of optimum hopper slide angle and viscosity to avoid heel formation at a fixed flow rate and curtain height, will work only at the specified height and flow rate. Heel formation can still be a problem if any of the parameters are changed, since the interaction of the parameters is unspecified. The method of the present invention identifies the relationship between all the key coating parameters and allows an a priori optimization of the coating conditions to avoid recirculation. Furthermore it allows a prediction of the likely effect on the recirculation boundary, if one or more of the coating parameters is changed.
The above and other objects, features and advantages of the present invention will become apparent from the following description of a preferred embodiment, in connection with the following drawings, in which;